Mode field specifies the scope of tracing, that is, global for all users on the system or user for users who have tracing enabled by the Administration API task function.
模式(mode)指定跟踪的范围，即是针对系统上的所有用户，还是只针对通过Administration API task函数启用了跟踪的用户。
A constant field in a class is similar to a global or function constant, except that it is scoped to the class and it can have various modifiers governing its visibility and usage.
类中的常数字段类似于全局或函数常数，不同的只是它的作用范围是该类，而且它可以用多个修饰符来指定其可见性和用法。
A variable field in a class is similar to a global or function variable, except that it is scoped to the class and it can have various attributes governing its visibility and usage.
类别中的变数栏位与全域或函式变数类似，但它只能在类别范围中作用，而且可以具有控制可视性与用法的不同属性。
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